Are you using the BSP toolchain to compile u-boot? It could be that some libraries are missing. Please provide more details on your configuration. You may find steps to build u-boot on the link below for reference.

I'm not sure if the toolchain you are using does have i.MX6 support but you should be able to download and use the above one or you may extract the toolchain from the i.MX6 BSP (which does take some time).